Assuming you have a DSLR (or some higher-end mirrorless camera), you can add an eyepiece accessory to provide additional magnification (or minification, if you need negative adjustment).

The adjustment in your viewfinder is measured in "diopters", just like an eyeglass prescription*, and as you note, the built-in adjustment can only go so far. But you can add additional correction as well with an accessory for most SLRs and at least Fujifilm and Leica mirrorless cameras. See [the selection at B&H](http://www.bhphotovideo.com/c/search?atclk=Eyepiece+Accessory_Diopters&ci=19005&N=4077634559+4111488783) for many options — use the filter on the right to narrow down to the camera and correction you need.

If this doesn't work for you, another option is a magnifier for the rear LCD screen; this works basically the same way, but since that is inherently larger and more flexible with workable distance from your eye, it may be more convenient if your vision is really reaching the limits.

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<sub>* for that matter, it also works [just like a supplementary close-up lens on the front of the camera for macro](http://photo.stackexchange.com/questions/59163/how-can-i-calculate-the-effect-of-a-supplementary-close-up-lens-a-k-a-macro-fil) — just applying to the viewfinder rather than to the image through the lens.</sub>